1. Field of the Invention
The present invention relates to fiber Bragg gratings and more particularly to a method for writing multiple waveguide grating filters at different center wavelengths.
2. Background Information
An optical transmission system transmits information from one place to another by way of a carrier whose frequency typically is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, or a lightwave signal.
An optical transmission system includes several optical fibers. Each optical fiber includes several channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength. One link of an optical transmission system typically has a transmitter, the optical fiber, and a receiver. The transmitter converts an electrical signal into the optical signal and launches it into the optical fiber. The optical fiber transports the optical signal to the receiver. The receiver converts the optical signal back into an electrical signal.
An optical transmission system that transmits more than one channel over the same optical fiber is sometimes referred to as a multiple channel system. The purpose for using multiple channels in the same optical fiber is to take advantage of the unprecedented capacity offered by optical fibers. Essentially, each channel has its own wavelength, and all wavelengths are separated enough to prevent overlap.
One way to transmit multiple channels is through wavelength division multiplexing, whereupon several wavelengths are transmitted in the same optical fiber. Typically, four channels are interleaved by a multiplexer, launched into the optical fiber, and separated by a demultiplexer at a receiver. Along the way, channels may be added or dropped using an add/drop multiplexer. Wavelength division demultiplexing elements separate the individual wavelengths using frequency-selective components such as optical gratings, which can provide high reflectivity and high wavelength selectivity with the aim of increasing the transmission capacity of optical fibers.
One such optical grating is a fiber Bragg grating, which selectively transmits or reflects specific wavelengths of light propagating within the optical fiber. A fiber Bragg grating is commonly a portion of an optical fiber that has a refractive index profile that varies periodically along the length of the optical fiber. Refractive index variations with a single period (Λ) selectively reflect light with a wavelength (λ) ofλ=2Λ  (Equation 1).
Other wavelengths are transmitted essentially unimpeded. Alternatively, the period (Λ) can be chosen to vary along the length of the fiber in order to reflect a broad range of wavelength, e.g. chirped gratings. Such broadband gratings can for example be used for dispersion compensation to provide a wavelength dependent time delay to a propagating signal with a finite bandwidth.
Simple periodic fiber Bragg gratings are known in the art, and many different methods have been described for fabricating fiber Bragg gratings. However, it is difficult to fabricate complex fiber Bragg gratings using existing techniques.